It's only 23 inches long, but one tyrannosaurus skull has been the subject of serious debate among paleontologists for decades.
In 1988, a group of researchers named it Nanotyrannus lancensis, suggesting that it was a different animal that lived in the shadow of the Tyrannosaurus rex. In 1999, another group argued that the skull and similar specimens were T. rex as a juvenile, before the species underwent the extraordinary growth spurt that preceded adulthood.
Over the years, the adolescent T. rex hypothesis has gained traction.
“Most people have bought into it, myself included,” said Nick Longrich, a paleontologist at the University of Bath in England.
But Dr. Longrich changed his tune. In A study released Wednesday In the journal Fossil Studies, he and colleagues argue that there is enough evidence to reconstruct Nanotyrannus. as its own species within the larger family of tyrannosaurs. Based on anatomical features, they argue that it is not particularly closely related to T. rex.
Other experts say the study is unlikely to end the debate.
“It's kind of like Schrödinger's dinosaur,” said Thomas Holtz, a paleontologist at the University of Maryland who was not involved in the work. “This paper will keep people talking about it, but it won't really solve it.”
To find out, Dr. Longrich's team studied the original 23-inch skull and the newer finds, named Jane and Patty, as well. A long-disputed tyrannosaurus specimen, “Dueling Dinosaurs”. They argued that all of these represent adolescent T. Rex, Dr. Longrich said. But his team said they found about 150 differences in their anatomy, including skull details; open, mouth-like murmur; and longer arms and claws than an adult T. rex.
He also said the specimens have features consistent with adult animals, not juveniles. Growth rings inside the bone from three samples—including Jane and Pat—also indicate a slowing of the growth rate. Researchers estimated that the animals weighed a ton more than the T. rex, which was between four and five tons.
“We have three individuals, which basically rules out individual variation or variation in growth,” Dr. Longrich said. “What we're seeing is that the growth patterns are not consistent with the juveniles of these animals.”
So where are the actual minors t. Rex? Dr Longrich believes he has found a fragment of one – a piece of skull from the University of California, Berkeley, collections described in the paper. “In every feature it was a T.rex,” he said.
Other paleontologists are not ready to dismiss the teenage T. rex hypothesis and have voiced strong opposition to the paper.
The considered specimens show common characteristics of adult T. rex — including the forehead, forehead and braincase, said Thomas Carr, the Carthage College paleontologist who first identified Nanotyrannus. represented young T. Rex. Moreover, he disagrees with the claim that they do not fit the growth pattern found in other tyrannosaur skulls.
“T. rex and tyrannosaurs in general, the differences between juveniles and adults are quite extreme and are easily dismissed by humans,” said Dr. Carr.
Holly Woodward, a paleontologist at Oklahoma State University who prepared the growth data used by Dr. Longrich's team, also rejected their findings. The spacing of the innermost growth rings in the bone tissue of a nearly full-grown adult T. rex suggests “small growth rates at a young age before a large growth spurt,” he said.
Dr Woodward added that the team's choice of mathematical models risked distorting the picture by showing young animals growing up, even if they weren't.
“I'm just not convinced that the growth curve arguments support that hypothesis,” he said.
Dr. Longrich countered that even the teenage T. rex proponents had not proven their case: “I would throw them into the camp and say, ‘Where's the evidence for your hypothesis?'
He explained that “for a nanotyrannus to turn into a T. rex would require an extraordinary amount of transformation.” No other dinosaur evolves like this, Dr. Longrich argues: Everything his team has studied fits neatly into the shape of a Nanotyrannus or T. rex.
Credible paleontologists have historically argued both sides of the issue, Dr. Holtz said. Part of the issue is that most T. rex specimens are adults, with only a few subadults. Everyone recognizes this flaw; They just don't agree on its meaning.
Discovery of an older Nanotyrannus or young T. rex, which is different from the Nanotyrannus shape, will help clarify things, Dr. Holtz said. So could future data on Jane and the Tyrannosaurus from “Dueling Dinosaurs.” Although the team's paper makes interesting suggestions, Dr. Holtz said it is not enough for him to rule out the hypothesis “that this is a juvenile T. rex.”
The debate continues. For an animal that may or may not exist, a Nanotyrannus is interestingly difficult to kill.